1. Field of the Invention
The present invention relates generally to supercharged compression ignition engines having an exhaust gas recirculation (EGR) and, more particularly, to a system for and a method of controlling a compression ignition internal combustion engine having an EGR system and a multiple supercharger system including a plurality of superchargers.
2. Description of the Background Art
Many compression ignition engines use turbochargers to improve engine performance. A turbocharger increases the density of the intake air into the engine. The higher density air increases the amount of fuel the engine may combust. As a result, the power output of the engine increases.
Turbochargers typically include a turbine and a compressor connected by a common shaft. The turbine has blades attached to a wheel, which is mounted on the shaft. A turbine housing encloses the turbine and connects to the exhaust manifold of the engine. The turbine housing has vanes for directing the exhaust gases against the turbine blades. The compressor has blades attached to another wheel, which also is mounted on the shaft. A compressor housing encloses the compressor and connects to the intake manifold of the engine. The compressor housing has vanes for assisting the compressor to pressurize intake air. The compressor housing is isolated from the turbine housing.
In operation, exhaust gases pass through the exhaust gas manifold into the turbine housing. The vanes in the turbine housing direct the exhaust gases against the turbine blades. The exhaust gas pressure causes the turbine to spin, which causes the compressor to spin. The spinning compressor pressurizes the intake air. As a result, higher density air is provided to the intake manifold.
In a turbocharger, the exhaust gas pressure has a direct effect on the intake air pressure. As the exhaust gas pressure increases, the turbine and consequently the compressor spin faster. A faster spinning compressor increases the intake air pressure. The opposite effect occurs as the exhaust gas pressure decreases.
Many conventional turbochargers have a fixed geometry. The vanes in the turbine and compressor housings are stationary. By design, a fixed-geometry turbocharger operates efficiently at a particular engine speed and load. Conversely, it operates less efficiently at engine speed and loads for which it is not designed.
At low engine speeds, the exhaust gas pressure is low. It may be below the minimum necessary for operating the turbine. As the engine accelerates from idle or slow speeds, there is a delay from the time when the engine load increases to the time when there is sufficient exhaust gas pressure to spin the turbine. Even when the turbine spins, the exhaust gas pressure may not reach a pressure high enough to spin the turbine as fast as it is necessary for the compressor to produce the desired intake air pressure.
The exhaust gas pressure increases as engine speed increases. At some point, the pressure becomes high enough to overpower the turbocharger. An overpowered turbocharger reduces engine performance.
To improve efficiency, fixed-geometry turbochargers are sized to provide high compressor speeds at low engine speeds. The vanes in the turbine housing are usually narrow to increase the exhaust gas pressure. The vanes also direct the exhaust gas flow toward a portion of the turbine blades. While these changes improve the performance of the turbocharger at low engine speeds, they adversely affect the performance of the turbocharger at high engine speeds. The narrowing of the vanes lowers the exhaust gas pressure at which the turbocharger becomes overdriven.
To avoid overdriving, fixed-geometry turbochargers have a waste gate or similar valve positioned between the turbine and the exhaust gas manifold. When the exhaust gas pressure reaches a certain level, the waste gate opens to divert exhaust gas away from the turbine.
New turbocharger designs have a variable geometry. Turbochargers of such designs are called variable geometry turbochargers (VGT). There are several designs for the variable geometry turbocharger. In one design, a movable sidewall varies the effective cross sectional area of the turbine housing. In another design, the turbine and/or compressor housings have variable nozzles. The nozzles move to change the flow area and flow direction. In many designs, only the turbine has variable nozzles.
A variable nozzle turbine (VNT) turbocharger typically has curvilinear nozzles, which rotate between open and closed positions about a pivot. In some designs, the closed position leaves a small gap between the nozzles. In other designs, the nozzles touch when they are closed, which essentially stops the flow of exhaust gases to the turbine. The nozzles connect to each other by a ring or similar apparatus to move in unison.
When the exhaust gas pressure is low, the nozzles close to create a narrower area for the exhaust gases to flow. The narrow area restricts gas flow through the turbine housing, thus increasing exhaust gas pressure. The nozzles also direct the exhaust gases optimally at the tips of the turbine blades. The directed flow and higher pressure enable the turbine to start spinning sooner and at a faster rate. As a result, a VNT turbocharger provides the high compressor speeds desired at low engine speeds.
As the exhaust gas pressure increases, the nozzles open to reduce the restriction to the gas flow. The gas flow also is directed toward the entire length of the turbine blades. With less restriction and broader gas flow, the turbine and consequently compressor spins slower than if the nozzles were closed under these conditions. As a result, the turbocharger is able to respond and correct for overdriven conditions. An optimal position for the nozzles is determined from a combination of desired torque response, fuel economy, and emission requirements.
Exhaust gas recirculation (EGR) systems are used to reduce NOx emissions by increasing the dilution fraction in the intake manifold. EGR is typically accomplished with an EGR valve that interconnects the exhaust manifold and the intake manifold. In the combustion cylinders, the recirculated exhaust gas acts as an inert gas, thus lowering the flame and in-cylinder gas temperature and, hence, decreasing the formation of NOx.
In compression ignition engines equipped with a VGT system and an EGR system, optimal engine performance in terms of fuel economy and emissions is achieved by coordinating the operation of two actuators.
EP 1 077 320 A2 (published Feb. 21, 2001), which was filed by the assignee to which the present application has been assigned, discloses a conventional VGT control system for a compression ignition internal combustion engine having an EGR system and a single VGT system. The conventional system utilizes a microprocessor-based controller having boost maps stored therein. As shown in FIGS. 70 and 72 of this published EP application, the boost maps contain the desired opening ratio for nozzles of a VGT as a function a combination of a first input parameter and a second input parameter. The first input parameter is an intake air amount equivalence value (tQas0). The second input parameter is an actual EGR rate (Megrd). The controller monitors the engine speed and accelerator pedal opening angle, and has maps stored therein to determine the first input parameter and second input parameter. This conventional system has proven to be satisfactory.
However, a need remains to improve the conventional system such that it is applicable to vehicles having a compression ignition internal combustion engine equipped with an EGR system and a supercharger system including a plurality of superchargers.
Accordingly, an object of the present invention is to provide a system for and a method of controlling a vehicle having a compression ignition internal combustion engine equipped with an EGR system and a supercharger system including a plurality of superchargers.